Tunable synthesis of renewable tricyclic high-energy-density fuels from lignin-derived oxygenates

High-energy-density (HED) fuels, such as widely used JP-10 and RJ-4, are critical for advanced aerospace applications due to their exceptional volumetric energy density. However, their production is heavily dependent on petroleum feedstocks, raising environmental concerns and supply security risks. Therefore, the development of renewable HED fuels that can rival the properties of petroleum-based HED fuels is imperative. Herein, we report a two-step protocol for producing tricyclic HED fuels from lignin-derived benzaldehyde and phenols. The initial condensation between benzaldehyde and phenol over Nafion at 80 °C affords tricyclic product in 89% yield. Nafion catalyst can be reused for five consecutive cycles with no appreciable loss in activity. Mechanistic investigations reveal that the condensation involves a rate-determining nucleophilic addition of phenol to benzaldehyde and a rapid second nucleophilic substitution with another phenol molecule. Subsequent hydrodeoxygenation is achieved via a decoupled strategy using Pd/C and H-Y zeolite at 220 °C, effectively preserving the tricyclic skeleton with 92% yield. Notably, lignin-based phenolic mixtures are also compatible with this two-step process, affording alkyl-substituted tricycloalkanes with density above 0.90 g/mL and freezing point below -50 °C. The introduction of alkyl side chains finely tunes fuel properties by depressing the freezing point without compromising the high energy density inherent to the tricyclic core. Life-cycle assessment and techno-economic analysis reveal that the adoption of renewable electricity and earth-abundant catalysts will reduce the carbon footprint and production costs of these fuels.